The ecology of rafting in the marine environment - Bedim

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Plankton Suspension- Feeders MARTIN THIEL & LARS GUTOW Small Predators Floating Item Herbivores Top Predators Figure 18 Simplified food web model for rafting community. attached micro-organisms. Consequently, primary production may be higher on and around floating items than in the surrounding water bodies. Smith et al. (1973) determined the respiration of the Sargassum community to be 2–5 times greater than the surface layer (1–2 m) plankton respiration. In summary, trophic processes are enhanced on floating substrata compared with the surrounding surface waters. The high import of dead organic matter (plant detritus and animal carcasses) is converted by the rafting community into living organic matter, partly sustaining the rafting community itself and being exported to the pelagic environment via associated fish predators (Figure 18). Thus, floating items appear to have a catalytic function at the sea surface, similar as has been reported for mussel beds and coral reefs in benthic habitats (Prins et al. 1998, Wild et al. 2004) or for particulate organic matter (POM) in the water column (Azam & Long 2001). Temporal and spatial variability of the rafting community Estimates of floating duration As a result of the observed ecological interactions and of growth of rafters, it can be expected that the rafting community experiences substantial changes during the journey. However, in order to reveal the pattern of the succession it is important to have reliable estimates of the floating duration, which is not a trivial task. Several approaches have been taken. Strong indication for temporal changes comes from experimental studies in which entire plants or parts of floating macroalgae were tethered in coastal habitats (reviewed in Thiel 2003c). Even though in these cases floating duration is exactly known, these studies suffer from the drawback that they are conducted in the vicinity of coastal source populations and the observed succession may not necessarily be representative of the open ocean situation. Other authors collected floating items at various distances from a potential source region, where distance may be used as an estimate of floating duration. Using this approach, Ingólfsson (1995) revealed a strong decrease in abundance of common rafting organisms with increasing distance from the shore. Also, Donlan & Nelson (2003) found that species richness on floating items (mainly wood) varied significantly with distance from the coast, 372 Carnivorous Plankton Epiphytes Plankton
RAFTING OF BENTHIC MARINE ORGANISMS but they noted that this relationship was weak. This may be due to the fact that distance from the next coast is not a good estimator for floating duration (e.g., Hobday 2000a), since the underlying assumption, namely that transport direction of floating items is away from the shore, may not be fulfilled. This is not necessarily the case since oceanic currents or wind direction may be along-shore in their direction or even push items towards the shore (for overview see Thiel & Gutow 2004). In some cases, time and place of introduction of a floating item is well known (e.g., anthropogenic items or volcanic pumice) and thus reliable estimates of floating duration can be obtained. Another method to estimate rafting duration is based on the size of particular rafters or the integrity of rafting substrata (e.g., Stoner & Greening 1984, Hobday 2000b, Tsikhon-Lukanina et al. 2001a). This approach is based on the same principle used by forensic scientists (e.g., Sorg et al. 1997), namely to utilise approximate growth rates of common indicator organisms/rafters (e.g., stalked barnacles) to reach minimum estimates of rafting duration. Assuming that stalked barnacles colonise items soon after these went afloat and combining this with maximum growth rates (see Table 17), one can obtain relatively reasonable estimates of floating duration. Many authors have used this principle and sizes of some rafters in order to make inferences about the minimum duration of floating (e.g., Hamada 1964, Horn et al. 1970, Jokiel 1984, 1990a, DeVantier 1992, Tsikhon-Lukanina et al. 2001a). Helmuth et al. (1994) used percentages of branch points of floating Macrocystis pyrifera plants colonised by stalked barnacles to estimate floating time. In addition to this measure, these authors also utilised blade length, a measure that has been further refined by Hobday (2000b). Ryland (1974) and Niermann (1986) used Sargassum colour and condition to obtain approximate age estimates. Stoner & Greening (1984) had used a combination of these last measures to categorise floating macroalgae in young, medium and old patches. Utilising all these different approaches to obtain estimates of the floating duration of common substrata and their rafting organisms, some consistent patterns of the temporal succession of the rafting community emerge. Species exchanges Successional changes of rafting community During the rafting journey, organisms frequently abandon, colonise, thrive or are suppressed on floating items. Depending on their biological characteristics, some species are capable of quickly colonising newly available floating objects, while the same species may not be able to withstand increasing competition or predation pressure on their rafts. Other organisms are poor colonisers, but once settled on a floating item they may be able to establish dense populations, excluding other organisms. Some floating substrata begin their journey with a diverse fauna, while others start in a clean state (most plastics and volcanic pumice). Macroalgae and mangrove trees that are detached from benthic habitats, carry with them many of their original inhabitants. Edgar & Burton (2000) observed that many invertebrates abandoned floating Durvillaea antarctica rapidly after detachment of the alga. However, the few species that were still present on the plants after one day of floating have a good chance to survive for longer periods adrift. Loss of original inhabitants from recently detached macroalgae was also reported by Edgar (1987), Kingsford & Choat (1985), and Vásquez (1993). Caprellid amphipods, for example, have been seen to descend back to the sea bed if detached Sargassum plants start to float to the sea surface (Takeuchi & Sawamoto 1998). Three main factors are responsible for this rapid loss of original inhabitants (Thiel 2003c): (a) they actively abandon floating items, (b) they cannot hold onto floating items, (c) they cannot resist the high predation pressure in the pelagic environment. It is thus not surprising that some of the most common organisms lost immediately after detachment are echinoderms, brachyuran crabs and large gastropods 373
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1 Oceanography and Marine Biology:
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RAFTING OF BENTHIC MARINE ORGANISMS
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Table 2 RAFTING OF BENTHIC MARINE O
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Table 2 (continued) Species Region
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Porifera RAFTING OF BENTHIC MARINE
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Cnidaria (Anthozoa) RAFTING OF BENT
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The ecology of rafting in the marine environment - Bedim

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